A need to detect aircraft induced wake vortices and associated turbulence is well established and is set out in a program in Europe named SESAR, http://www.sesarju.eu/, and in a similar program in North America called NextGen, http://www.faa.gov/nextgen/. These two programs are summarised in the ICAO ASBU document, http://www.icao.int/sustainability/Pages/ASBU-Framework.aspx. In order to meet the objectives of these programs there is a need for a technology and systems that can detect the position and strength of aircraft induced wake turbulence in real time and in all weather conditions. In particular there is a need for a system that can detect wake turbulence in the approach or departure corridors of an airport, being critical regions wherein a following aircraft encountering wake turbulence from a leading aircraft can suffer an upset which can have severe consequences.
The wake from an aircraft arises because of displacement of air required to generate lift. This wake can evolve into several forms, all of which are potentially hazardous for following aircraft. The wake can roll up into a vortex which may last from 20 seconds to several minutes depending on atmospheric conditions. The vortices can have a variety of states which may include core vortices with core diameters of around 1 m that rotate at between 80 and 200 m/s and which may be wrapped into large core vortices with a diameter up to 30 m that rotate at between 5 and 20 m/s. The vortices always have strong air turbulence or wind shear associated with them which can disrupt the flight of an aircraft encountering it. Sometimes the aircraft induced wake may evolve directly into turbulence without going through a vortex stage. Thus any process to measure safety of an approach corridor should be capable of detecting all aircraft induced wakes. Since turbulence is always associated with an aircraft wake then a reliable arrangement is to directly detect wake turbulence. However, detecting aircraft induced wake vortices alone may not be sufficient to ensure that an approach corridor is safe for a following aircraft from turbulence induced or generated by a leading aircraft.
Various approaches to detecting aircraft induced wake vortices have used RADAR, LIDAR and SODAR wherein the aim is to detect position of wake vortices so that they can be tracked. There have also been several attempts to measure strength of aircraft induced wake vortices, some have been successful and some not so successful. A summary of these efforts is given in:
http://ntl.bts.gov/lib/33000/33700/33701/WakeNet3_Europe_March2010_Sensor_Volpe.pdf.
The above efforts have not yielded a system that can reliably detect all types of aircraft induced wake turbulence in real time and in all weather conditions as is required for an operational system to deliver the benefits outlined in the SESAR, NextGen or the ICAO ASBU programs.
A known aircraft vortex system outlined in U.S. Pat. No. 3,671,927 uses refraction of sound waves for detecting wake vortices but can detect only a few of the vortices present. This occurs because the technique can only detect one of the pair of aircraft induced vortices that have a rotational velocity greater than about 80 m/s (http://ntl.bts.gov/lib/46000/46000/46025/Burnham_CharacteristicsWake_VortexTracking.pdf) and consequently cannot also detect slower vortices or turbulence associated with an aircraft wake which is also present. Thus the system in U.S. Pat. No. 3,671,927 may not provide a reliable indication of aircraft induced wake vortices and/or turbulence.
A known acoustic wind sensor also outlined in U.S. Pat. No. 3,671,927 has limitations in that it can only detect vortices that are in the beam of a single transmitter by using a scanning receiver beam and there is thus a much lower probability of detection than is required for an operational system. A similar system is outlined in “Doppler Acoustic Vortex Sensing System by R. P. McConville” in: http://www.dtic.mil/dtic/tr/fulltext/u2/a062026.pdf, wherein both pulsed and CW acoustic systems for detecting aircraft wake vortices are described. In particular FIG. 3-34 shows Backscatter Pulsed, Forward scatter Pulsed and Forward Scatter CW systems. A pictorial diagram of a Forward Scatter CW system is given in FIG. 4-1. A CW system inherently has no range information, however range can be obtained by sweeping transmitter and/or receiver beams.
Previous attempts to detect wake vortices using a chirp based SODAR outlined in Applicant's U.S. Pat. No. 7,284,421 entitled Detection of wake vortices and the like, and U.S. Pat. No. 7,703,319 entitled Characterisation of Aircraft Wake Vortices, consisted of refraction of an acoustic signal having a pulse compression waveform (chirp) by the core of the vortex. This system gave accurate measurements of vortex height providing that the vortex core rotated at a velocity of greater than 80 m/s, but could not reliably detect vortices that rotated at less than 80 m/s or turbulence associated with an aircraft wake because the backscattered signal-to-noise ratio was too low to be detectable and because no reliable information on strength or decay time of all aircraft induced wake turbulence could be obtained. Poor detection of all aircraft related turbulence in the flight path was due to a single transmitter antenna only partially illuminating the area above the antenna with sufficient transmitted signal to obtain the necessary signal-to-noise ratio and coverage width of the flight path to provide a useful “synthetic aperture”. Thus U.S. Pat. No. 7,284,421 fails to detect all aircraft generated wake vortices and turbulence in a “synthetic aperture” that is necessary to determine whether the flight path is safe for a following aircraft.
Chirp based SODAR disclosed in Applicant's U.S. Pat. No. 6,755,080 entitled Acoustic Sounding, U.S. Pat. No. 7,317,659 entitled Measurement of the Air Characteristics of the Lower Atmosphere and U.S. Pat. No. 7,178,408 entitled SODAR Sounding of the Lower Atmosphere, has 45 dB more gain than a conventional short pulse SODAR and 25 dB more gain than chirp SODAR systems disclosed in other prior art. The latter systems use each amplitude measurement independently at an update rate of the chirp SODAR to provide a wake turbulence measurement. The latter systems improve on the prior art by using a chirp signal format with relatively high chirp rates (eg. at least 1000 Hz/second), by using IIR window linear phase filters in a matched filter receiver, and by using an improved antenna design. Chirp acoustic signals belong to a class of waveforms known to be suitable for use in a pulse compression or matched filter receiver. A good introduction to pulse compression waveforms and matched filters is given in “Introduction to Radar Systems, Third Edition, by Merrill I. Skolnik, McGraw Hill, 2001, ISBN 0-07-118189-X.
An improved antenna design includes double side walls with internal acoustic absorbers for maximum isolation as well as offset feed antennas with relatively low side lobes. The antenna design also includes a relatively high efficiency compression driver enabling sound levels up to 135 dBa to be transmitted. This may enable the chirp SODAR to measure substantially all types of atmospheric turbulence in real time without averaging, a capability that was not previously available to any SODAR, LIDAR or RADAR system. Measurement of turbulence may be achieved when acoustic energy is backscattered from small scale temperature discontinuities and/or fluctuations in the atmosphere.
Safe aircraft apertures for arrival and departure have been previously discussed in: http://www.transportresearch.info/Upload/Documents/201208/20120807_184001_3122_ATC_Wake%20D2_12.pdf.
The aperture disclosed in the latter reference is in the context of showing how modelled wake vortices may be depicted in terms of the aircraft corridor, although no suggestion is given to applying the concept to real time measurement of wake turbulence or the method to implement such real time measurement system. For example, U.S. Pat. No. 7,284,421 discloses a “synthetic aperture” but does not disclose how to build a system with a non-synthesised aperture.
Many attempts have been made over the past 30 years to produce a predictive system based on wake vortex behaviour modelling for estimating aircraft wake vortex position and demise times. However, some sections of the aviation community have questioned the viability of predictive/inferential information in place of real-time measurements of the vortices. Thus as far as applicant is aware, there is no known acoustic pulse compression system that indicates how to measure aircraft wake turbulence in an aperture in real time for the purpose of obtaining safe aircraft spacing within an approach or departure corridor.
The term turbulence as used herein denotes small-scale, irregular air motions characterised by winds that vary in speed and/or direction. Turbulence is significant because it mixes and churns the atmosphere and causes water vapour, smoke, and other substances, as well as energy, to become distributed both vertically and horizontally.
The term aperture as used herein denotes a slice or cross-section along an approach or departure corridor near an airport. The aperture contains a region within which an aircraft generates or induces turbulence and through which a following aircraft passes after a leading aircraft induces wake turbulence. The term aperture may also denote the whole of an area within which wake turbulence induced by an aircraft may travel between different aircraft corridors including approach and departure corridors.
The table below shows that chirp based SODAR is a technology that can meet the requirements of an aircraft wake turbulence measurement system. LIDAR or RADAR solutions cannot achieve anywhere near the level of performance set out in the table because of resolution and update rate limitations. The update rate in the table is important because sufficient time resolution is required to ensure that turbulence in an aperture is clear so that aircraft spacing can be optimised.
Aperture Aircraft Wake Turbulence Measurement Requirements TableUpdateMinMaxCoveragerateResolutionAveragingRangeRangeWidthRequirements<5 Sec5 mNo10 m150 m150 mAveragingShort Pulse20 sec10 m Averages40 m 50 m150 mSODARto getWithdataarrayChirp SODAR<5 sec2 mNo10 m250 m150 mAveragingwith arrayused
Most work to date has concentrated on measuring aircraft wake vortex trajectories and then trying to develop models of this behaviour. This has not been successful because many parameters required to describe wake vortex behaviour are highly variable and difficult or impossible to obtain in practice.
The wake vortex problem is complex precisely because a large number of parameters are involved. Setting aside various operational scenarios, the problem involves parameters introduced by vortex-generating aircraft, by vortex-encountering aircraft, and by the intervening atmosphere. The vortex is initially characterized by parameters of the vortex-generating aircraft including weight, wingspan, speed, flap and spoiler settings, proximity to the ground, engine thrust, lift distribution, etc. The encounter (safe or hazardous) is characterized by parameters of the following aircraft including speed, wingspan, roll control authority, phase of flight, etc. Meteorology variables including wind, crosswind, atmospheric stability, background atmospheric turbulence, etc. also plays a critical role in determining how long a vortex may remain hazardous.
Current separation times between aircraft arriving and departing airports have been set conservatively to minimize turbulence encounter. The settings are generally conservative and rely on estimated demise or transit of turbulence out of departure or arrival corridors within a separation time, without any actual measurement of position or strength of wake turbulence being made within the corridors.
The present applicant recognizes that there is considerable potential to reduce aircraft separation times on arrival and departure if it was possible to determine in real time how long it takes for the wake turbulence from a leading aircraft to reduce to a safe threshold within the approach or departure corridor. Determination of a safe threshold for aircraft wake turbulence is relatively complex as different aircraft can tolerate different levels of turbulence. The determination may be further complicated by presence of normal or background atmospheric turbulence which itself is highly variable. Thus to correctly measure an actual safe turbulence threshold requires a prior knowledge of the state of the atmospheric background turbulence as well. Determination of a safe threshold for aircraft wake turbulence in real time may lead not only to better capacity and utilization of airport infrastructure but also to improved safety.
The method and system of the present invention may measure atmospheric turbulence in an aperture including turbulence generated or induced by an aircraft in the aperture, a means of determining different types of turbulence in the aperture and a demise time, being a time taken for the aircraft generated or induced turbulence to fall to or below a set or safe level or threshold within the aperture. If the demise time is measured in real time, then a reasonable safety or time buffer or margin may be applied to determine a safe or optimum spacing time for a following aircraft to pass through the aperture.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission or a suggestion that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.
Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.